The present invention relates to a method for forming an ohmic electrode on a substrate made of silicon carbide having a large band gap, and also relates to a semiconductor device in which a transmission amplifier required to exhibit high-output characteristics and a reception amplifier required to exhibit low-noise characteristics are integrally formed on a single substrate made of silicon carbide.
In recent years, semiconductors composed of silicon carbide (SiC) (hereinafter, simply referred to as "SiC semiconductors") have become an object of much attention as next-generation semiconductors, because the SiC semiconductors are advantageous in physical properties in view of the wide band gap thereof and because there are a substantially infinite amount of constituent elements for the SiC semiconductors. Since the SiC semiconductors have a crystalline structure formed by covalent bonds, the physical properties thereof are very stable. In addition, since the SiC semiconductors have a large band gap and a high melting point, heat treatment is required to be conducted at a high temperature in order to form an ohmic electrode on a substrate made of an SiC semiconductor.
Hereinafter, a conventional method for forming an ohmic electrode by conducting a heat treatment at a high temperature will be described as the first prior-art example with reference to FIGS. 6(a) to 6(c) . FIGS. 6(a) to 6(c) show cross-sectional structures illustrating the sequential process steps of a conventional method for forming an ohmic electrode in a semiconductor device. First, as shown in FIG. 6(a) , metal films 102 made of Ni or the like are formed on the upper surface of a substrate 101 made of SiC. In this state, an ohmic contact is not formed between the metal films 102 and the substrate 101, but a Schottky contact is formed therebetween.
Next, as shown in FIG. 6(b) , the substrate 101 is inserted into the gap between heating coils 103a provided on the upper inner surface of a radio frequency heating oven 103 and heating coils 103a provided on the lower inner surface of the radio frequency heating oven 103, and then a heat treatment is conducted on the substrate 101 at a high temperature in the range from about 1000.degree. C. to about 1600.degree. C. As a result, an ohmic contact is formed between the metal films 102 and the substrate 101 because the metal-semiconductor interface between the metal films 102 and the substrate 101 is turned into an alloy. Consequently, ohmic electrodes 102A are completed as shown in FIG. 6(c) . This method is disclosed, for example, by C. Arnodo et al., in "Nickel and Molybdenum Ohmic Contacts on Silicon Carbide", Institute of Physics Conference Series Number 142, pp. 577-580, 1996.
On the other hand, in recent years, remarkably downsized and performance-enhanced cellular phones have been rapidly popularized. Such a rapid popularization has resulted not only from the development of performance-enhanced batteries but also from the development of high-performance field effect transistors, gallium arsenide (GaAs) MESFETs in particular. A GaAs MESFET is a high-performance switching device exhibiting such excellent radio frequency characteristics as to attain various advantages such as low-voltage operation, high gain, high efficiency, low noise, low distortion and the like, and thus is used universally as a transmission/reception amplifier for portable terminal units including cellular phones. Recently, thanks to tremendous development in cutting-edge technologies, a conventional hybrid IC is on the verge of being replaced by a newly developed microwave monolithic IC (MMIC) in which both a reception amplifier section exhibiting low-noise characteristics and a transmission amplifier section exhibiting high-output characteristics are formed integrally on a single chip.
Hereinafter, a conventional transmission/reception amplifier having an MMIC structure will be described as the second prior-art example with reference to FIG. 10.
FIG. 10 shows a cross-sectional structure of a conventional MMIC in which a transmission amplifier and a reception amplifier are formed integrally on a single chip. As shown in FIG. 10, a high-output amplifier section 112 for transmission and a low-noise amplifier section 113 for reception are formed in a GaAs substrate 111 such that these sections are spaced from each other via a certain gap. The high-output amplifier section 112 is constituted by a MESFET having a relatively large gate width, while the low-noise amplifier section 113 is constituted by a MESFET having a relatively small gate width. See, for example, K. Fujimoto et al., "A high Performance GaAs MMIC Transceiver for Personal Handy Phone System (PHS)", 25.sup.th European Microwave Conference, Proceedings, Vol. 2, pp. 926-930, 1995.
However, the conventional method for forming an ohmic electrode, identified above as the first prior-art example, has various difficult problems to solve. Firstly, in accordance with the method, since a heat treatment is conducted at as high a temperature as the growth temperature of SiC crystals in order to form the ohmic electrodes 102A, damage is possibly done on the substrate 101. Secondly, in order to conduct a heat treatment at such a high temperature, a special apparatus such as the radio frequency heating oven 103 is required. Thirdly, in order to optimize the conditions during the heat treatment process, it is very difficult to control a temperature and an environmental gas. And finally, in order to monitor the safety against such a high-temperature process, the control/monitoring procedure becomes adversely complicated.
On the other hand, the MMIC, identified above as the second prior-art example, has the following problems. In the MMIC, GaAs, used as a material for the substrate 111, has a relatively low thermal conductivity of about 0.5 W/cm.multidot.K. Thus, if the output of the high-output amplifier section 112 is to be further increased, then the temperature of the substrate 111 is adversely raised. As a result, the low-noise characteristics of the low-noise amplifier section 113, resulting from the high electron mobility (=about 6000 cm.multidot.cm/Vs) of GaAs, are disadvantageously deteriorated. In consequence, it has heretofore been difficult to implement an MMIC of high-output type providing an output of several watts to several hundreds watts.